A COMPOSITE POLLEN DIAGRAM EROM CO. MEATH, IRELAND BY G. F. MITCHELL

Transcription

1 [.257] A COMPOSITE POLLEN DIAGRAM EROM CO. MEATH, IRELAND BY G. F. MITCHELL Trinity College, Dublin (With one pull out diagram) PRELIMINARY Prof. Knud Jessen of Copenhagen visited Ireland in the summers of 1934 and 1935 to investigate the quaternary deposits, especially the peat deposits in which the country is so rich. Examination of the samples collected had not been completed by 1939, and the full results of Prof. Jessen's investigations cannot be available in the British Isles until after the war. A preliminary scheme of Irish forest development has been published and a series of zones into which Irish pollen diagrams may be divided indicated (Jessen, 1937). In recent years the study of post-glacial deposits has gone on energetically in England, and the purpose of the present paper is to make available (pending the publication of Prof. Jessen's material) a pollen diagram from the east of Ireland which appears to be typical for that part of the island. Dr Godwin has drawn up a general scheme of forest development in England and Wales based on pollen diagrams covering the whole post-glacial period (Godwin, 1940). The diagrams have been divided into a series of zones. Unfortunately, the notation of the Irish zones does not coincide with that of the English zones, though the zones themselves (based as they are on stages of forest development) are largely in agreement. In the present paper the word 'phase' will be used as a synonym for 'zone'. In this way the confusion, which it is hoped will be removed by bringing the Irish notation into agreement with the English, will not be increased. The word 'pollen-horizon' will be used to describe a level in a pollen diagram defined by a certain relation between the tree pollens. THE DI.'\GRAM The existence of a crannog in a drained lake basin to the east of Dunshaughlin, Co. Meath, was first noted over a century ago (Wilde, 1840). The crannog was e.xcavated by the Harvard Archaeological Mission in 1934, 1935 and 1936, and at the same time the deposits in the basin were examined by the author, principally by a series of borings across the basin running through the crannog. The author is much indebted to Dr H. O'N. Hencken and the other members of the American Mission for assistance in the field. An account of the stratigraphy (illustrated by pollen diagrams) has already been published (Mitchell, 1940). The country in the vicinity of the basin, which lies at about 350 ft. O.D., is built up of undulating ridges of boulder clay rich in limestone. The deposits in the basin are chiefiy lacustrine, but marginally there is a superficial layer of fen peat. None of the borings suggested that trees of the 'carr' type had ever invaded the margin of the fen, and the pollen diagrams seem to be free of major local distortions, though the slopes surrounding the basin were doubtless well wooded.

2 258 G. F. MITCHELL The pollen diagrams indicated that the rates of deposition in different parts of the basin had varied widely. While no single diagram showed a continuous record from the early post-glacial period till ca, A.D when the basin was drained, each stage of forest development was illustrated by at least two diagrams, between which there was always a close agreement. It seemed therefore that a composite diagram giving the complete record could be prepared by arranging certain shorter diagrams serially. As the rate of deposition throughout the period had not been constant, it was obvious that it would be misleading to relate the vertical height (and consequently apparent duration) of each phase in the diagram to the thickness of deposit that had accumulated at the site of the particular samples chosen to represent it. Two alternatives suggested themselves. The first was to allot to each phase the same vertical height with the consequent disadvantage that the duration of each phase would appear to be the same, which is certainly not the case. The second was to assume that certain well-marked features in Irish post-glacial development were at least approximately contemporaneous with similar features in Scandinavia, to arrange the Meath pollen-horizons corresponding to these features at vertical distances from one another proportional to the time that appears to have separated the features in Scandinavia and to construct the diagram round this framework. The second alternative was adopted. The features selected were (i) the late-glacial solifluction deposit, assumed to be contemporaneous with the upper Dryas clay of Denmark (Jessen & Farrington, 1938); (ii) the increase of Corylus to a maximum, assumed to be contemporaneous with the beginning of the Boreal maximum of northwest Europe (Erdtman, 1928); (iii) the end of the Corylus maximum, assumed to be contemporaneous with the beginning of the Baltic Littorina transgression (Jessen, 1940); and (iv) the abrupt and widespread appearance of fresh Sphagnum peat, assumed to be contemporaneous with the Grenzhor'izont of the bogs of north-west Europe (Jessen, 1934). The corresponding pollen-horizons were arranged at the same distances from one another as separate the features in Jessen's scheme of post-glacial development in Denmark (Jessen, 1939). The late-glacial horizon lies at the base of the diagram (Fig. i), the other pollen-horizons are indicated by horizontal lines, and the top of the diagram corresponds with the date A.D The analyses falling between any two of the selected pollen-horizons were arranged at equal distances from one another. This spacing makes it appear that the spacing of the samples (from which the analyses were prepared) differed at different levels; 10 cm. was the maximum interval between any two samples. The composite diagram was prepared from the following diagrams of the original paper; Fig. 5 (in part). Fig. 6, Fig. 8 (in part), and Fig. 9. The extent to which assumption and adjustment have ahered the original data may be seen by comparing the composite with the original diagrams. Other examples of the adjustment of pollen diagrams to an arbitrary length may be seen in a paper by Rudolph (1930). DISCUSSION Irish diagrams from late-glacial deposits have already been discussed (Jessen & Farrington, 1938); as the corresponding deposits in England have yet to be elucidated, the lateglacial phases have been omitted from the diagram, which begins at the beginning of the post-glacial period proper. The first phase is marked by the dominance of Betula, Pinus and Salix being the only

3 A composite pollen diagram from Co. Meath, Ireland 259 other trees represented; non-tree-pollen (not shown in the diagram) is abundant. The phase is referred to as Zone IV in England and Wales (Godwin, 1940) and in Ireland (Jessen, 1937). In Ireland Pinus tends to fall as the phase progresses, and Jessen has suggested {in litt.) that this may be due to over-representation of Pinus at the beginning of the phase due to transport of this pollen from a distance. In England, on the other hand, Pinus tends to rise, especially in the south and east. The beginning of the next phase is marked by the appearance of Corylus pollen in considerable amounts. Corylus increases rapidly throughout the phase. In England the rise in Pinus continues, and in the south and east it displaces Betida from the dominant position; in the north and west the increase in Pinus is less marked, while in Ireland, though there is a tendency for Pinus to rise to a maximum, the position of Betula is never seriously challenged. Salix is reduced and the N.T.P. retreats very markedly. The phase, known in England as Zone V and in Ireland as Zone Va, is brought to a close with the appearance of Ulmus in quantity. At the same time there is frequently a Pinus maximum. In the next phase Ulmus rises rapidly to importance and Corylus is maintained at a very high level. In Ireland values over 1000% are regularly recorded. Quercus appears in quantity. Betuta undergoes a marked reduction and Pinus also falls back. Salix retreats to an insignificance from which it never again emerges. The phase is known in England.as Zone Via and forms in Ireland the lower part of Zone Nb. The rise of Quercus to a maximum marks the next phase. Zone V\h in England and a later stage of Zone V6 in Ireland. Ulmus and Corylus fall back slightly from their previous values and Betula and Pinus continue at low values. At the end of the phase Ulmus climbs once more, but Corylus commences a rapid fall. The next phase is marked by a pronounced minimum in Corylus, a reduction in Ulmus and Quercus, a marked maximum for Pinus and the appearance of Alnus in some quantity. The phase is recognized in England as Zone Vic but has not hitherto been recognized in Ireland where it occurs at the transition between Zone \b and Zone VI. Alnus climbs steadily to a position of importance and throughout the next phase (Zone VII in England and Zone VI in Ireland) Alnus, Quercus, and Ulmus maintain considerable values, while Betula and Pinus are of lesser importance. The climatic optimum falls within this phase, and in the absence of Tilia, Carpinus and Fagus it is difficult to trace well-defined phases in the subsequent deterioration of the Irish forests. At a time earlier than that marked by the sudden renewal of growth in the raised-bogs Pinus virtually disappears from Irish diagrams. The disappearance is often preceded by a small but well-defined Pinus maximum (not seen in the Meath diagram). It is probable that this temporary increase in Pinus is the equivalent of the Pinus maximum that marks the beginning of the English Zone VII/VIII. Jessen has pointed out (in litt.) that in Ireland the subsequent reduction in Pinus is usually associated with a maximum for Quereus (sometimes accompanied by a rise in Betula), while Alnus and sometimes Ulmus are reduced. Such a horizon is clearly seen in the Meath diagram bringing the Irish Zone VI to an end. A similar horizon is suggested in diagrams from Shropshire (Hardy, 1939) and Wales (Godwin & Mitchell, 1938; Hyde, 1940). The horizon lies below the renewal of the growth of Sphagnum peat. In the latest phase of several Irish pollen diagrams there is a pronounced rise in Betula, but neither the beginning of the rise nor its completion has as yet been closely linked with the behaviour of the other tree pollens. In his Irish material Jessen recognizes a Zone

4 26o G. F. MITCHELL VIII in which Betula has returned to importance. In England a similar rise in Betula occurs and the associated behaviour of Carpinus and Fagus enables a reasonably precise horizon to be established; here Zone VII/VIII ends and Zone VIII begins. Comparing the post-glacial forest development in the east of Ireland, as typified by the Meath diagram, with the corresponding development in England and Wales, it is apparent that as long as the post-glacial climate was improving forest development in each area followed closely parallel courses, doubtless in response to the climatic factor. But as soon as the optimum had been passed the courses ceased to be parallel, and each became much less clearly defined. Climatic deterioration may have allowed an added importance to local factors. Godwin has made a similar observation on the relation between the forest history of England and Wales and that of the Continent (Godwin, 1934). In the English Zone VII, immediately the optimum has been passed Ulmus shows a considerable reduction in quantity; in the east of Ireland no similar movement can be traced. In England there is some consistency in the later movements of Ulmus; in Ireland no such consistency has as yet been established. In England Pinus sinks to a low level before Zone VII is far advanced; in Ireland it is often maintained at considerable values till a later period, only sinking to insignificance a short time before the renewal of growth in the raised-bogs. In both areas there is a late return of Betula to importance. In England the return is accompanied by the persistent appearance of Carpinus and Fagus:, in Ireland no such guides are available. It is fortunate that Carpinus and Fagus never rose to positions of importance in the forests of England and Wales, for had they done so the small movements of the other trees would have been completely submerged. Detailed study of the late minor oscillations in both areas may yet enable correlation to be carried further forward. For the same reason it is fortunate that Tilia (except in East Anglia) only formed a minor component of the English woodlands. In England and Wales pollen of this tree does little more than mark the attainment of and subsequent decline from the climatic optimum. ARCHAEOLOGICAL HORIZONS The crannog itself and two other isolated finds make it possible to insert three archaeological horizons in the composite diagram. The crannog, which is thought from the archaeological evidence to have been occupied from the latter part of the eighth to the latter part of the tenth century A.D., belongs to a period later than the return of Betula to importance. A Late Bronze Age settlement in Co. Ofi^aly dated between the fourth and first century B.C. preceded the return of Betula to importance (Hencken, 1942). A bronze spearhead, typologically not earlier than Middle Bronze Age and perhaps considerably later, lies immediately above the horizon contemporaneous with the renewal of growth in the raised bogs (Fig. 12, Mitchell, 1940). No Late Bronze Age objects have yet been dated in Ireland to horizons preceding this renewal of growth, and six Late Bronze Age objects have been dated to horizons succeeding the renewal of growth. A sherd of pottery with Neolithic 'A' affinities bccurs at the horizon where Pinus dwindles to insignificance (Fig. 11, Mitchell, 1940). Owing to the slowness of deposition at the point where the sherd was found, its exact position in the composite diagram is a matter of difficulty: it cannot be older than the position shown. A few Neolithic, Early Bronze and Middle Bronze Age objects have been dated to horizons preceding the renewal of growth in the raised bogs. The pollen-horizons associated with finds of these

5 NR. DUNSHAUGHLIN. id. PERCENTAGES OF TOTAL TREE POLLEN, EXCLUDING SALIX AND CORYLUS O 4O O 5O 9O O 50 O 400 5O 80O 30 O LOO 2OO 3OO 4OO 50O I I 1 I I I I I 1 I I p I 1 1 I r 1 i I ; I I I r( i i F ' I CRANNOG 1732 PINUS PINE BETULA BIRCH ULMUS ELM I I L QUERCUS OAK ALNUS ALDER SALIX WILLOW CORYLUS HAZEL Fig. I. A composite pollen diagram from near Dunshaughlin, Co. Meath, extending from the beginning of the post-glacial period till ca. A.D. I8OO.

6 A composite pollen diagram from Co. Meath, Ireland 261 three types are not widely separated, and it is probable that this apparently rapid succession of archaeological types is a reflexion of a slow accumulation of deposit in the period preceding the Grenzhorizont. SUMMARY A composite pollen diagram covering the whole post-glacial period has been constructed from a series of diagrams based on samples from near Dunshaughlin, Co. Meath. The construction of the composite diagram is described. The phases into which the diagram may be divided are compared with the zones already established in Britain and in Ireland. Until the post-glacial climatic optimum is reached the diagram is in agreement with diagrams from Britain. The agreement with diagrams from the north and west of Britain is particularly marked. After the post-glacial optimum had been passed the agreement is less marked though the underlying movements continue to be similar. Three archaeological horizons are shown in the diagram. REFERENCES ERDTMAN, G. (1928). Studies in the post-arctic history of the forests of north-western Europe. I. Investigations in the British Isles. Geol. Foren. Stockh. Fork. 50. GODWIN, H. (1934). Pollen analysis. An outline of the problems and potentialities of the method. II. General applications of pollen analysis. New Phytol. 33. GODWIN, H. (1940). Pollen analysis and forest history in England and Wales. New Phytol. 39. GODWIN, H. & MITCHELL, G. F. (1938). Stratigraphy and developnient of two raised bogs near Tregaron, Cardiganshire. Nezu Phytol. 37. HARDY, E. M. (1939). Studies in the post-glacial history of British vegetation. V. The Shropshire and Flint Maelor mosses. Neiv Phytol. 38. HENCKEN, H. O'N. (1942). Ballinderry Crannog No. 2. Proc. Roy. Irish Acad. 47, C. HYDE, H. A. (1940). On a peat bog at Craig-y-llyn, Glam. Data for the study of post-glacial history. IV. New Phytol. 39. JESSEN, K. (1934). Preliminary report on hog investigations in Ireland, Irish Nat. J. 5. JESSEN, K. (1937). Contribution to MAHR, A. Proc. Prehist. Soc. Paper II. JESSEN, K. (1939). Naturforholdene og Mennesket i Danmarks Oldtid. Fortid. og Nutid. 13. JESSEN, K. (1940). Contribution to Movius, H. Proc. Roy. Irish Acad. 46, C. JESSEN, K. & FARRINGTON, A. (1938). The bogs at Ballybetagh, near Dublin, with remarks on late-glacial conditions in Ireland. Proc. Roy. Irish Acad. 44, B. MITCHELL, G. F. (1940). Studies in Irish quaternary deposits: Some lacustrine deposits near Dunshaughlin, Co. Meath. Proc. Roy. Irish Acad. 46, B. RUDOLPH, K. (1930). Grundzlige der nacheiszeitlichen Waldegeschichte Mitteleuropas. Beih. Bot. Zhl. 47. WILDE, W. R. (1840). On the animal remains and antiquities recently found at Dunshaughlin, Co. Meath. Proc, Roy. Irish Acad. 1.

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